The present invention is directed to a method of making a sheeted crosslinked cellulose and the product resulting from the process. The invention is especially directed toward a crosslinked cellulose sheet which can later be easily reslurried in water without excessive fiber breakage.
Crosslinked cellulose products have been described in the chemical literature for many years. These products are normally made by reacting a material, usually bifunctional, that will tie together hydroxyl groups on neighboring cellulose chains. Formaldehyde and various derivatives of urea have been the crosslinking agents which have received the greatest study. However, many other materials which have actual or latent bifunctional reactive groups have also been reported.
Crosslinked celluloses are of great commercial importance in the textile industry where they are widely used for the production of wash-and-wear and other wrinkle-resistant types of fabrics. Crosslinked cellulose fluff has also been described for use in disposable absorbent products such as diapers. Here advantages is taken of the fact that crosslinked fibers are normally stiffer than their untreated counterparts. The fluff products formed from these fibers are of somewhat lower density (or greater bulk) and tend to hold retained liquid better under compressive forces encountered during use of the product.
While the advantages contributed to disposable absorbent products by crosslinked cellulose fibers are real, products using these crosslinked fibers have never become commercially important. This is apparently because of the difficulty of making a sheeted crosslinked fiber product that can be later refiberized at the point of use without creation of an excessive amount of fines. Unfortunately, crosslinking also results in considerable fiber embrittlement. Additionally, most of the crosslinking agents which have been used serve to give both chemical and physical bonding between adjacent fibers in the sheets. This, in addition to the increased fiber brittleness, has made mechanical wet or dry defiberization of sheeted crosslinked pulps impractical. In an effort to overcome this problem, various workers have considered treating sheeted pulp with a latent crosslinking material, fluffing, and then carrying out the crosslinking reaction by heating the cellulose fluff. An example of this is seen in Bernardin, U.S. Pat. No. 3,224,926. Van Haaften, Canadian Patent 806,352 treats loose fibers with a crosslinking material and catalyst. These moist fibers are then expanded into a loose fluffy condition and cured.
The stiffness of crosslinked cellulose fibers can add desirable properties to certain sheeted pump products. Here it is typical to use only a portion of crosslinked fibers in the ultimate product. Attempts to do this have encountered the same problems mentioned earlier. If a product is crosslinked in sheeted form, it becomes very difficult to redisperse without serious fiber breakage by normal wet repulping processes employed in paper mills. As noted before, there are two apparent reasons for this. The strength of a sheeted cellulose product is developed in part by mechanical entanglement of the fibers but, much more so, by hydrogen bonding in those areas where fibers overlap are in intimate contact with each other. This hydrogen bonding develops only when the fibers are dry, in a crosslinked sheeted product, when the crosslinking reaction is normally carried out by heating after the sheet has been fully dried, two phenomena can occur. One of these is interfiber crosslinking. The reaction occurs in areas of intimate fiber-to-fiber contact and serves to chemically bind the fibers together. Perhaps of even greater importance, many of the crosslinking materials that also form thermosetting adhesives under the heated conditions used in the crosslinking reaction. Scanning electron micrographs of heated dimethylolurea treated fibers show many small spherical nodules of ureaformaldehyde resin on the surface and within the fiber lumen. These nodules serve to adhesively bond adjacent fibers so that it is very difficult to separate them under any conditions without considerable fiber breakage. Because the crosslinked fibers tend to be so brittle, the fibers the themselves will often break leaving the bonded areas between adjacent fibers intact. There is a related side issue to this phenomenon. It is still an unresolved question as to how much of the crosslinking reaction is a surface phenomenon as opposed to an internal one.
Earlier workers in the field have also tried to deal with the problem of making a sheeted cellulose pulp product containing only a portion of crosslinked fibers. As one example, Bernardin, in U.S. Pat. No. 3,434,918, treats sheeted fiber with a crosslinking agent and catalyst. This is then wet aged to insolublize the crosslinker, so-called "wet fixation." This wet aged fiber is then redispersed before curing. The redispersed fiber can be mixed with untreated fiber and the mixture sheeted. The final product is then heat cured. In a variation of this process the same inventor, in Canadian Patent 813,616, heat cures crosslinked fibers as a fluff and then mixes this product with conventional papermaking fibers.
These mixtures of crosslinked fibers with untreated fibers are potentially useful for making products such as filter media, tissues, and towelling where high bulk and good water absorbency are desired without excessive stiffness in the product. Freimark et al, in U.S. Pat. No. 3,755,220, describe making a soft, high wet strength sheet, although this does not use crosslinked fibers. These inventors utilize well known debonders or softeners with cationic wet strength resins to gain an increase in the ratio of wet to dry tensile strength, usually without serious loss in absolute values of wet tensile strength. The debonder itself can be cationic or anionic and may be added to the papermaking stock prior to or following the addition of the wet strength resin.
In U.S. Pat. No. 4,204,054, Lesas et al describe spraying unsheeted bulk fibers with a solution of formaldehyde, formic acid and hydrochloric acid. These fibers are then immediately dispersed in a hot air stream at about 170.degree.-200.degree. C. 1-20 seconds. This appears to give primarily surface area crosslinking without serious effect on fiber flexibility. The inventors note that 10-40% of these fibers can be mixed with conventional fibers to give a sheeted product with good flexibility and water absorbency.
Unfortunately, the problems encountered handling bulk fibers; i.e., those in individual loose form as opposed to a sheeted product, have been so great as to be commercially nearly insurmountable to the present time. The fiber must be dried by flash drying or some similar procedure where it is usually suspended in a hot air stream. The dried fiber is then baled or bagged. Because of the very short fiber length, compactly packaging a loose fiber form of wood pulp is technically very difficult and expensive. An alternative procedure, where the loose fibrous product might be prepared at the ultimate point of consumption, has been even more unattractive and has met with a wall of resistance by potential consumers.
The reader who might be interested in learning more detail of the chemistry of cellulose crosslinking can refer to any of the standard texts on cellulose. One resource which treats the subject quite thoroughly is by Tersoro and Willard in Cellulose and Cellulose Derivatives, Bikales and Segal, eds., Part V, Wiley-Interscience, New York, (1971), pp. 835-875.
Reference was made to use of fiber debonders, also called sheet softeners in the earlier comments relating to U.S. Pat. No. 3,755,220. These materials can be generally classified as surfactants which are applied to the fiber while it is still wet, before any hydrogen bonding has occurred. Most typically they are cationic in nature, based on quaternary ammonium compounds which have one or more fatty substituents. Although not as commonly used, nonionic and anionic types are also commercially available, Frequently a combination of a cationic and nonionic type may be employed. These products are widely used within the pulp and paper industry and are commercially available from a number of supplies. Similar products are used in the textile industry.
Debonders serve to make a softer sheet by virtue of the fatty portion of the molecule which interferes with the normal hydrogen bonding. They are quite commonly used in the manufacture of fluff pulps which will be later converted into absorbent products such as disposable diapers. The use of a debonder can reduce the energy required to produce a fluff to half or even less than the required for a nontreated pulp. This advantage is not obtained without a price, however, Many debonders tend to reduce water absorbency as a result of hydrophobicity caused by the same fatty long chain portion which gives the product its effectiveness. In order to overcome this problem, some manufacturers have formed adducts of ethylene or propylene oxide in order to make the products somewhat more hydrophilic. Those interested in the chemistry of debonders will find them widely described in the patent literature. The following list of U.S. patents provides a fair sampling, although it is not intended to be exhaustive: Hervey et al., U.S. Pat. Nos. 3,395,708 and 3,554,862; Forssblad et al, U.S. Pat. No. 3,677,886; Emanuelsson et al., U.S. Pat. No. 4,144,122; Osborne, III., U.S. Pat. No. 4,351,699; and Hellsten et al., U.S. Pat. No. 4,476,323. All of the aforementioned patents describe cationic debonders. Laursen, in U.S. Pat. No. 4,303,471, describes what might be considered a representative nonanionic debonder.
U.S. Pat. No. 3,844,880 to Meisel, Jr. et al. describes the use of deposition aid (generally cationic), an anionic resin emulsion, and a softening agent which are added sequentially to a pulp furnish to produce a soft product having high wet and dry tensile strength. The opposite situation; i.e., low wet tensile strength, is preferred for a pulp which is to be later reslurried for some other use.
Croon et al., in U.S. Pat. No. 3,700,549, describe a cellulose fiber product crosslinked with a polyhalide, polyepoxide, or epoxyhalide under strongly alkaline conditions. Epichlorohydrin is a preferred material. In their examples Croon et al teach the use of their treated fiber in absorbent products such as diapers and sanitary napkins. All of the crosslinking materials are insoluble in water. Croon et al teach three methods to overcome this problem. The first is the use of vigorous agitation to maintain the crosslinking agent in a fine droplet-size suspension. Second is the use of of a polar cosolvent such as acetone or dialkylsulfoxides. Third is the use of a neutral (in terms of being a nonreactant) water soluble salt such as magnesium chloride. In a variation of the first method a surfactant may be added to enhance the dispersion of the reactant phase. After reaction the resulting product must be exhaustively washed to remove the necessary high concentration of alkali and any unrelated crosslinking materials, salts, or solvents. The method is suitable only for cellulosic products having a relatively high hemicellulose content. A serious deficiency is the need for subsequent disposal of the toxic materials washed from the reacted product. The Croon et al material would also be expected to have all other well known disadvantages incurred with trying to sheet a stiff, brittle crosslinked fiber.
Steiger, in U.S. Pat. No. 3,658,613, teaches a method for "wet" crosslinking a sheeted pulp product so that it can later be defiberized dry with reduced fiber breakage and knot content. The reaction is carried out with a crosslinker such as formaldehyde, ureaforms, glyoxal and halohydrins under very strongly acidic conditions; e.g., in a reaction medium containing 50% by volume of 37% hydrochloric acid, for about 30 minutes. The pulp after reaction must be neutralized by a mild alkali and washed prior to drying. The strongly acidic reaction conditions and difficulty of supporting a saturated wet sheet for the necessary operations over the required period of time would pose almost insuperable barriers against commercial production.
U.S. Pat. No. 3,819,470, Shaw et al. teach the reaction of a "substantive compound" with cellulose to form a pulp product which can later be refiberized in water. A debonder may be used at various points of addition in the process. The substantive compound is apparently a cationic polymer, such as a cationic ureaform wet strength agent. The inventors clearly teach against the use of crosslinking agents and offer a number of reasons why their use would not be suitable in the process. Low retention of the polymer, only about 50%, would seriously affect the economics of the process and create a serious waste disposal problem.
Chung, in U.S. Pat. No. 3,440,135, teaches the manufacture of a crosslinked cellulose product which is then later dried in loose fiber form. Sheeted pulp is treated in a bath of a crosslinking agent such as dimethylolurea. A surfactant may optionally be included to improve wet dispersion of the pulp before the drying step. The inventor notes the critically of wet aging the treated product in the range of 16-48 hours before it is refiberized and dried in a hot air suspension. This aging requirement would virtually doom the commercial possibilities for the process since it would impose on the mill the requirement for keeping a large inventory of wet sheeted pulp. The additional process expense and space requirements would make the cost of the product prohibitively high.
To the knowledge of the present inventor, no one except Chung as noted above used a debonder with a cellulose pulp which is also treated with a crosslinking agent. One skilled in the art would not expect this to be an effective combination, i.e., they would expect the interfiber bonding propensities of the crosslinking agents to completely overpower any advantage in the reduction of wet or dry strength that might be contributed by the debonding agent. The deficiencies that make the Chung process commercially impractical have already been noted.